The invention relates to a method and apparatus for a repeatable handback attempt after an inter-mobile switching station (MSC) handoff during an active call. If a preceding handback was not successful, the method and apparatus prepares for another handback attempt and, if a handback is called for, attempts to handback the call again.
While the invention is particularly directed to the art of handback attempts after an inter-MSC handoff, and will be described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used in conjunction with any type of handoff involving subsequent teardown of a trunk between a target and source after completion of the handoff.
By way of background, handoff and handover are synonyms referring to an operation in wireless technology that transfers communication with a mobile station (MS) from a first base station (BS) to another BS which may be in a different MSC. After the inter-MSC handoff, circumstances may occur that cause communications with the MS to be transferred back to the first MSC. This is referred to as a handback or a handoff back because it follows the previous handoff. Thus, as used herein, handoff and handover have the same meaning and may be used interchangeably. Likewise, handback and handoff back have the same meaning and may be used interchangeably. Moreover, since a handback is a certain type of handoff that follows a previous handoff, a handback may also be referred to as a handoff or handover.
Inter-vendor handoffs (i.e., hard handoffs) use ANSI-41 messages between the MSCs of two different vendors. A vendor may also use ANSI-41 messages for handoffs between its own MSCs (i.e., intra-vendor handoffs). Subsequent to a given handoff, for example from MSC A to MSC B, the mobile user may travel back to the area of MSC A. This creates a situation where a handback attempt is requested where MSC A (the source for the previous handoff) is now a target for the handback attempt. After a successful handback, the resources on the MSC B (the source for the handback attempt) are cleaned up. If a handoff or handback attempt fails, the MS returns a candidate frequency search report message to the source cell (i.e., BS). Upon receiving the candidate frequency search report message instead of a handoff complete message, the source BS communicates an abort message to the source MSC. This instructs the source MSC to abort the failed handoff or handback attempt. However, currently, there is no means of communicating failure of the handback attempt from the source MSC to the target MSC at that stage. Thus, there is no means for the source MSC to instruct the target MSC to abort the handback attempt. The handback attempt is only aborted by the target MSC after timers associated with the handback attempt on the target side expire before the handback attempt is completed.
It is important to note that (handoff and) handback attempts occur when the signal between the source cell and the MS is weak. Thus, it is desirable that the next (handoff or) handback attempt can follow soon after a failed attempt, otherwise the call could drop. On the other hand, it is not desirable to attempt the next (handoff or) handback before the allocated resources on the target side from the previous failed attempt are cleaned up or are about to be cleaned up. Since it is likely that there will be many simultaneous (handoff or) handback attempts by many MSs in that area, any strategy that attempts the next (handoff or) handback prior to clean up would potentially tangle the resources on the target side.
One problem associated with the forgoing scenario is that there is currently no ANSI-41 message for the source MSC to communicate to the target MSC that tells the target MSC to abort the handback attempt because the MS failed to re-tune to the target cell (i.e., BS) in the other MSC. Another problem is that the existing ANSI-41 FacilitiesRelease message cannot be used for this purpose as it contains the mandatory InterMSCCircuitID parameter which implies that an inter-vendor trunk between the source and target MSCs would be released by the recipient (target) MSC. However, in the case of a return from failed handback we want to keep this trunk and only release resources on the target MSC.
The present invention contemplates a new and improved handback process in a wireless telecommunication system after an inter-MSC handoff during an active call to an MS that resolves the above-referenced difficulties and others.